Plasma display panels emit light by exciting a phosphor with ultraviolet light generated by plasma discharge. Their configuration consists of the formation of a phosphor on a back substrate and enabling light emitted from the phosphor to be visible from a front substrate. Since phosphors have a milky white color ranging from white to pale gray when not emitting light, in an environment in which ambient light attributable to indoor light is radiated onto the panel, the milky white phosphor irradiated by the ambient light is observed from the front substrate, and both light resulting from phosphor emission and reflected light in the phosphor caused by the ambient light are both present, thereby causing a decrease in contrast.
FIG. 1 is a cross-sectional view of a plasma display panel of the prior art that prevents decreases in contrast. A plasma display panel 1 has a back substrate 2 that has a fluorescent layer 5, and a front substrate 3 that has a display electrode (not shown), both the substrates 2 and 3 are arranged in mutual opposition with a discharge space 4 interposed therebetween, and has a filter 6 having a low light transmittance a on the front side of the panel 1.
As a result of providing the filter 6, the quantity of display light 7 emitted by the fluorescent layer 5 of the panel 1 is attenuated by passing through the filter 6 due to the low light transmittance thereof. However, indoor light or other ambient light 9 also passes through the filter 6 and ambient light 10 that has passed through the filter 6 radiates onto the fluorescent layer 5, and as a result thereof, reflected light 11 again passes through the filter 6 whereby reflected light 12 appears on the outside. In other words, display light is attenuated once (α) by the filter 6, while ambient light 9 is attenuated twice (α2) by the filter 6. Accordingly, contrast, which is the ratio of ambient reflected light to display light, is improved as a result of providing the filter 6. However, the quantity of display light 8 itself is also attenuated by the filter 6.
Patent Documents 1 and 2 disclose configurations that prevent such decreases in contrast. In Patent Document 1, a dark-colored, band-shaped light-shielding film is formed in an area of an inverse slit between pairs of band-shaped display electrodes extending in the horizontal direction, and a phosphor on a back substrate is prevented from being visible through the inverse slit area. However, since a transparent electrode is formed in an emission area where electrical discharge between the pair of display electrodes occurs, and the phosphor on the back substrate is visible through the emission area, therefore, there are limitations on the degree to the contrast decrease prevention. In Patent Document 2, pairs of display electrodes extending in the horizontal direction are composed of metal bus electrodes and transparent electrodes, a third electrode extending in the horizontal direction is formed in a discharge area between the transparent electrodes, and it is proposed that a phosphor within the discharge area be shielded from ambient light by increasing the width of the third electrode in the discharge area between the transparent electrodes. The third electrode is held to a ground potential during panel driving, and together with operating as an auxiliary electrode that assists surface discharge of pairs of display electrodes, shields the phosphor in the discharge area from ambient light, thereby improving contrast.
In addition, in Patent Document 3, together with composing a pair of display electrodes with only metal bus electrodes instead of forming a transparent electrode that can result in high costs, the metal bus electrodes are composed of a plurality of electrode portions extending in the horizontal direction and linking portions that link the electrode portions in the manner of a ladder structure. The object of this Patent Document 3 is not to improve contrast.
Patent Document 1: Japanese Patent Application Laid-open No. H9-129142
Patent Document 2: Japanese Patent Application Laid-open No. 2006-202627
Patent Document 3: Japanese Patent Application Laid-open No. 2007-5297
There are expectations for the development of a plasma display panel capable of improving contrast in comparison with the example of the prior art and Patent Documents 1, 2 and 3 explained above. In Patent Document 1, there are limitations on improvement of contrast since there is no shielding of ambient reflected light in an emission area formed by discharge between a pair of display electrodes. In addition, in Patent Document 2, only a portion of an emission area is shielded with an auxiliary electrode, and since a fluorescent layer on a back substrate is still visible from the side of a front substrate in other portions of the emission area, considerable improvements in contrast cannot be expected. In addition, in Patent Document 3, a configuration for improving contrast is not described.